Suicide Risk Among US Veterans With Military Service During the Vietnam War

This cohort study examines the incidence and patterns of suicide from 1979 to 2019 among veterans who were deployed to the Vietnam War and veterans who served in the military at that time but were not deployed to the Vietnam War.


Statistical Analysis Testing Cox Model Assumption
The underlying assumption of the Cox proportional hazards model, the relative hazard remains constant over time with different predictor or covariate levels, was tested graphically using SAS PROC LIFETEST to plot the log [log(Survival Probability)] ) where the y-axis is log [-log(Survival Probability)] and the x-axis is log(time). 31It was further assessed statistically using SAS PROC PHREG to calculate and test whether the Schoenfeld residuals are correlated with time or with some function of time, which is one of several prescribed statistical analyses for evaluating the violation of the Cox model assumption. 32

Source of Expected U.S. Population Rates
Expected U.S. population mortality rates were those available from the National Institute for Occupational Safety and Health (NIOSH).The NIOSH data are based on death certificate data submitted to the National Center for Health Statistics by state vital statistics offices.These data include vital status and cause of death, as well as date of birth, date of death, sex, and race.The expected data for the U.S. population incorporates ICD codes stratified by sex, race, and 5-year calendar time periods from 1960 through the most current year available1 .The actual calculations of the expected rates are based on indirect standardization, where .the rates from the referent group (U.S. population) are stratified by age, sex, and race.These strata specific rates are then multiplied by the number of people in the corresponding strata of the population of interest.

SMR Calculation
Where the variables are defined as follows: • Ri1 = the stratum-specific rate in the observed cohort (the exposed cohort) • Ri0 = the stratum-specific rate in the unexposed population • Wi = stratum-specific person years in the exposed cohort.
Note that the ratio of observed to expected deaths is equivalent to a ratio of sums of weighted rates, in which the weights for each stratum are the person-years in the exposed group.The numerator is a weighted sum of the observed rates in the exposed population and the denominator is a weighted sum of the rates in the unexposed population.

SMR, 95%, C.I.
We calculated 95% CI for the SMRs using Byar's approximation as follows: where D is the observed number of deaths in the cohort and Zα/2 represents the 100(1-α/2) percentile of the standard normal distribution. 33ude Rate= (# deaths/pop at risk) X 100,000 population at risk Hazard rates (Crude rates presented over time) are the number of suicides observed for a specific calendar year divided by the number of all those alive up to and including that calendar year per 100,000 persons.)For veterans hazard rates were calculated by number of years since entry to follow-up (01/01/1979 for all veterans and calendar year (1979-2019) for U.S. males.Given all veterans were followed from 01/01/1979, number of years since entry to follow-up is roughly equivalent to calendar year.
As indicated in the formulas above the primary difference between CRs and SMRs is that CRs uses those alive thru a specific calendar date as a denominator, whereas SMR uses an expected value that is standardized to the cohort of interest.

Missing Data for Other Covariates
Only 4 theater suicides (.02%) and 152 (.2%) non-theater suicides were missing sex.All suicides had age, as this was available from NDI.

Deficits of Suicide Among Veterans Compared to Expected Based on U.S. Population (SMRs)
The decreased risks of suicide among theater, non-theater, and all Vietnam-era veterans when compared to the expected based on the U.S. population may be related to not having mortality data for the first five years or more after the veteran either left the military or left Vietnam.The decreased risks of suicide might also be due to the "healthy soldier effect", where health screening to serve in the military and access to medical care either from DOD or VA may create a healthier cohort than the U.S. population. 1

Comparison of SMRs to Crude Rates Overtime
As noted, the data indicate that based on the Standardized Mortality Ratio (SMR) analysis, the Vietnam era veterans both collectively and when stratified by in-theater status, are at a decreased risk for suicide compared to the U.S. population.Conversely, the individual calendar year crude rates (CRs) for suicide are consistently higher among the veteran cohorts compared to that for the U.S. population (Figure 2).The latter finding is consistent with that reported by the VA in their annual suicide prevention reports, where adjusted rates of suicide among all veterans are compared to suicide rates for the U.S. population.The findings based on the SMR analyses and the veteran to U.S. population crude suicide rates comparison by calendar year raise the question, Can a cumulative estimate of suicide risk among veterans compared to U.S. population (SMR) show no increased risk of suicide, while the individual calendar year for suicide rates are consistently higher among veterans compared to U.S. population?In fact, both findings can be true.The reason is due to features inherent in the calculation of the different suicide risk estimates.